1. Field of the Invention
The present invention relates to plasticating screws, and more particularly, to an improved mixing section for a plasticating screw.
2. Description of the Prior Art
The screw extrusion process involves feeding a solid, particulate polymeric material into the hopper of an extruder, or injection molding machine and then through a zone where the solid polymer is melted, or plasticized, into a viscous phase then forced into a die or mold of a desired configuration. The polymer is continuously propelled along a screw in a cylindrical bore through regions of high temperature and pressure.
One of the most important features of plasticating extruders is the screw. It is responsible for forwarding, melting, pressurizing and homogenizing the material from the feed hopper to the mold or die. The screw is typically divided into three major sections: the feed section which picks up the material from the hopper and propels it forward; the transition, or melt section, which melts the material; and the metering section which contributes to the uniform flow of the melted material at a sufficient pressure to force the material into the mold or die. Often, polymer not adequately melted in the transition section will melt somewhat more in the metering section.
Incomplete melting of the polymer results in a poor quality extrudate containing entrapped solid particles. Geyer U.S. Pat. No. 3,375,549 (the Geyer Patent), which issued on Apr. 2, 1968, discloses a screw having two channels, a solid channel and a melt channel, which continuously separate the viscous material from the solid material. The depth of each channel varies to permit the viscous material to pass forward while exerting pressure on the solid material to melt it.
Dray et al. U.S. Pat. No. 3,650,652 (the Dray Patent), issued on Mar. 21, 1972. The Dray Patent, like the Geyer Patent discussed above, discloses a screw having a first flight defining a main helical channel that extends through the length of the screw. A second flight begins downstream from the first flight at about the beginning of the transition section. Together, the first and second flights define an auxiliary helical channel. The second flight ends at about the beginning of the metering section. As the polymer melts, it passes from the main channel to the auxiliary channel. The melted polymer from each channel is combined in the metering section.
In addition to mixing and conveying, the metering section of plasticating screws provides the necessary pressure to force the extrudate into the mold or die. The rotation of the screw, in most applications less than 150 RPM, but in some up to 500 RPM, provides mechanical energy which is imparted to the polymer in the form of a shearing force. The application of shear is converted to thermal energy, resulting in a temperature rise in the polymer. Thus, the higher the shear, the higher the temperature.
In the metering section, the shear is often at its maximum level, thus, the average temperature of the polymer is raised. While the higher shear rates provide better mixing, the higher temperature can either deteriorate the polymers or cause processing problems downstream of the screw.
Kruder U.S. Pat. Nos. 3,870,284; 4,015,832; 4,173,417; and 4,277,182 disclose variations of a multichannel wave screw. Each flow channel is characterized by an undulating screw root which defines a plurality of crests and valleys. Solid material passes over the crests, thereby maximizing the melting of that material. Melted material passes with minimal additional shear. The polymer is thus uniformly mixed and melted without increasing the average temperature of the polymer.
A wide variety of extrusion screws of different designs have been developed to address specific problems in providing a satisfactory extrudate. Gregory U.S. Pat. No. 3,788,614 discloses an extrusion screw having an intermediate section of increased diameter. A plurality of concave channels of uniform depth are provided in the intermediate section in an interlocking helical arrangement.
Saxton U.S. Pat. No. 3,006,029 discloses a screw having a major helical channel defined by a helical arrangement of minor flights and minor channels. The minor channels form grooved passages between adjacent sections of the major channel. The channels are supposed to prevent the overheating and subsequent degradation of the plastic.
Brand U.S. Pat. No. 4,052,038 and Maillefer U.S. Pat. No. 4,085,461 disclose grooved passages in the mixing and melt sections, respectively, of extrusion screws. The passages are designed to decrease the temperature of the plastic to prevent degradation.
The object of the present invention is to provide a plasticating screw having an improved mixing portion which will uniformly mix the polymer, eliminate residual unmelted polymer and discharge it with the desired amount of pressure while permitting greater temperature control to avoid the degradation or overheating of the polymer.